I just got my hands on a PowerBook G4 1.25ghz with 768mb, and even though somewhat sluggish (opening the preferences window takes 4-5 seconds or so), aMule does seem to run stable on it. I haven't tried downloading a lot, but the problems you described were regarding the basic functionalities of aMule anyway.
So I'm thinking now, what you are experiencing must be caused by something else than just the speed of your cpu (btw., what is the exact speed anyway?). The most likely candidate is old settings that for some weird reason make aMule choke.
So, could you please shutdown aMule and try renaming/deleting the file "org.amule.aMule.plist" in "~/Library/Preferences/"? Just to be on the safe side, you might also want to rename/delete the folder "~/Library/Application Support/aMule" again. IIRC, those are the only locations where aMule stores its settings on OS X.
Please let us know if this fixed your issues or not. If not, we will have to do a little more digging.